1. Field of the Invention
The present invention relates to an echo cancellation apparatus in a digital mobile communication system and method thereof, and more particularly to an echo cancellation apparatus incorporated in a single chip together with a speech coder, which can improve convergence speed by using an excited signal as an input to an adaptive filter and also obtain a stabilized convergence characteristic while using an NLMS algorithm and method thereof.
2. Description of the Prior Art
Typically, an echo canceler has its object to effectively cancel echo generated by impedance mismatching which exists upon conversion of 4 terminals/2 terminals at a hybrid mating circuit located in the public switching telephone network (PSTN).
The echo canceler in the existing code division multiple access (CDMA) digital mobile communication system uses another DSP chip to perform an echo cancellation function or uses a single DSP chip in which a speech coder and an echo cancellation function is incorporated to perform a speech coding function and an echo cancellation, depending on the type of the digital signal processor (DSP) chip used in the speech coder. The echo canceler used in this mobile communication system uses an adaptive filter, in which an adaptive algorithm has been found in lots of application fields since 1960""s. Though many algorithms such as LMS, RLS, LSL, IIR filter structure etc. have published until now, NLMS (Normalized LMS) algorithm which has a minimum amount of computational complexity has been widely implemented in the field of echo cancellation.
Study made of the adaptive filter algorithm until now has been centered at the analysis study on the algorithm itself, and the technical development of the echo canceler mostly includes the contents for its implementation. The initial echo canceler is centered at a PCM speech communication, and the technology concerning the solution of problem in which a low transfer rate speech coder is combined to occur in a mobile communication system environment is rare. Nationally, although the technical development in a speech echo cancellation and a dual communication detection algorithm has proceeded, the technical development combined with the mobile communication environment has not been made.
Also, in a typical mobile communication system, though the NLMS (Normalized Least Means Square) algorithm has been widely used due to its easy implementation, the NLMS algorithm has a problem that it could not rapidly converge a great change in an initial state or an echo path because its convergence rate becomes slower if the correlating degree between the signals such as a speech signal in characteristic is high. RLS (Recursive Least Square) or LSL (Least Square Lattice) algorithms etc. have disadvantages that they have a rapid convergence rate but require a great amount of computation, thus making a realtime implementation difficult.
It is an object to provide an echo cancellation apparatus in which an echo canceller is combined with a speech coder at a single DSP chip in a mobile communication system, which can rapidly converge a great change in an initial state or an echo path because its convergence rate becomes slower with respect an input in which the correlation degree between a speech signal is high; which has an improved performance in cancelling an echo by lowering Eigenvalues of a speech signal; which has a simplified structure using an echo cancellation algorithm with an improved performance using a given amount of computation and a given memory; and which can provide an economic advantage by improving the performance through an algorithm without implementing additional hardware.
To achieve the above object, the echo cancellation method in a mobile communication system according to the present invention comprises the steps of storing codebook indexes and gains in a memory every frame using a normalized least means square (NLMS) in a mobile communication system; generating the stored codebook indexes and gains as an excited signal depending on a counter signal of a speech coding means; and adaptively filtering to cancel echo using the excited signal as an input signal of an adaptive filter.
To achieve the above object, the echo cancellation apparatus implemented with a speech coding means using a signal processing chip in a base station controller in a mobile communication system according to the present invention comprises adaptive filtering means for generating an excited signal using codebook indexes and gains as inputs depending on a counting timing from a speech coding means, and means for cancelling echo using the excited signal.